The present invention relates to a hydraulic chain tensioner having a piston longitudinally movable in a housing and more particularly to an air vent for such a hydraulic chain tensioner.
Tensioning devices, such as hydraulic tensioners, are used as a control device for power transmission chains as the chain travels between a plurality of sprockets. In an automotive application, the tension of the chain can vary greatly due to the wide variation in the temperature and the linear expansion among the various parts of the engine. Moreover, wear to the chain components during prolonged use can produce a decrease in the tension of the chain. As a result, it is important to impart and maintain a certain degree of tension to the chain to prevent noise, slippage, or un-meshing of the chain teeth. It is especially important in the case of a chain driven camshaft in an internal combustion engine to prevent the chain from slipping because the camshaft timing can be misaligned by several degrees, possibly rendering the engine inoperative or causing damage.
One example of a hydraulic tensioner is described in Kimura et al., U.S. Pat. No. 4,708,696. Kimura et al. disclose a hydraulic ball-type check valve tensioner having a plunger slidably fitted into a chamber and biased outward by a spring to provide tension to the chain. Fluid flows from a reservoir through a clearance formed between the ball and the seat of a check valve. The hydraulic pressure from an external source, such as an oil pump or the like, flows into a chamber through passages formed in the housing easily moving the plunger outward easily by the combined efforts of the hydraulic pressure and the spring force.
When the plunger tends to move in a reverse direction, the ball is tightly contacted with the ball seat to restrict flow of fluid from the chamber. A small clearance between the plunger and the housing wall permits fluid to escape to allow the plunger to retract. In such a fashion the tensioner achieves a so-called no-return function, i.e., movements are easy in one direction (outward) but difficult in the reverse direction (inward).
One drawback to a ball-type check valve is that its response time, i.e., the time to open or close to or from a full flow condition, is somewhat limited. In contrast, a spring check valve, such as that disclosed in Paul Jr., U.S. Pat. No. 3,896,834, provide a more rapid response time than a ball type check valve to open or close to or from a full flow condition.
A potential problem with hydraulic tensioners, however, is that they may not always maintain a predetermined tension, particularly when the engine is idling or at rest with low or no oil pressure. Unless appropriate oil pressure is applied to the chamber or the chamber is filled with sufficient oil, the plunger moves easily in both directions and loses the no-return function.
One solution to this potential problem is to provide a ratchet to act as a mechanical no-return device. For example, Suzuki, U.S. Pat. No. 4,874,352, provides a ratchet on the outer surface of the plunger to ensure the plunger will remain extended outward even upon low oil pressure or against tension from the chain. As another example, Renold, Great Britain Patent No. 1,018,211, provides an internal ratchet having a helical channel.
A problem with each of these types of hydraulic tensioners is that the fluid pressure inside of the hydraulic bore may change due to the introduction of air into the bore. If an excessive amount of air is present on the chamber, the plunger will easily move due to the compressibility of the air. As a result, the proper functioning of the tensioner will not be achieved. It is therefore desirable to provide for venting of air from the hydraulic bore.
One method for venting air is disclosed in U.S. Pat. No. 4,507,103 where a plug having a groove machined onto its flank is pressed into a smooth bore at the top of the piston so that a connection exists between the atmosphere and the fluid reservoir. A drawback to this method is that the plug and groove must be properly machined to fit into the smooth bore and to provide a suitable connection.
The vent of the present invention, on the other hand, is a simple and inexpensive apparatus to provide an air vent for a hydraulic tensioner. The vent is a disc having at least one channel formed on at least one surface to provide a connection between the atmosphere and the fluid reservoir. In this way excess fluid as well as air can be removed from the chamber.